Typical raster imaging systems for electrophotographic printers use a start-of-scan (SOS) signal to indicate when the laser beam is at a known position within the scanline. The SOS signal asserts when the laser beam has stimulated a detection or position sensor's photoelectric material beyond a characteristic threshold level. Once the imaging system receives the SOS signal, it then relies on a known time-to-distance relationship to properly position the image data across the scanline.
Raster imaging systems such as laser printers often feature several different darkness levels to meet the printing requirements of a wide variety of customers. One way to achieve these different darkness levels is to vary the current through the laser, which in turn varies the laser's output power. However, changes in the laser power will affect the scan position of the laser with respect to when the SOS signal is asserted by the SOS photosensor. The greater the laser power, the earlier the SOS signal is asserted (or output) because the SOS sensor's photoelectric material reaches the detection threshold sooner. This effect results in the image shifting toward the start of the scan, because the imaging system sees the SOS signal when the laser beam is positioned earlier in the scanline than it was before any increase in the laser power. This shift is not noticeable in systems that rasterize in a single direction, because every scan will shift in the same direction and by the same amount, thereby preserving the scan-to-scan alignment. However, this type of shift may be noticeable in bi-directional raster systems because the shift for scans rasterized during the laser beam's reverse travel is in the opposite direction as compared to scans that are rasterized during the laser beam's forward travel. This results in a misalignment between the forward and reverse scans of the image.